A 58-year-old woman began to show ataxia at age 45 and dysarthria at age 56. Neurological examination revealed slurred speech, truncal ataxia, and pyramydal sign. Neither history of alcoholism nor hereditary factors were found. The level of serum ammonia was increased. Brain MRI study showed a high signal intensity in the cerebral peduncle and globus pallidus and mild cerebellar atrophy on T1-weighted image. A portal-systemic shunt due to a shunt vessel was found between the left splenic and kidney veins although she did not show any other symptoms or signs due to liver cirrhosis. Her symptoms dramatically improved by an intravenous administration of branched amino acid. The present case suggests an importance in finding a treatable cerebellar ataxia.